The ideal geometric configuration of a four wheel vehicle is a rectangle in which: the steerable wheels will run parallel with each other and are equidistant from the center of the connecting axle or its equivalent; in which the non-steerable wheels will run parallel with each other and are equidistant from the center of the connecting axle or its equivalent; in which the non-steerable wheels either track with the steerable wheels or are equally off set from the steerable wheel tracks; and in which the vehicle body has its longitudinal geometric center line coincident with the longitudinal center line for the steerable and non-steerable wheels.
The practical and economic considerations involved in the production of wheeled vehicles take into account the complications in connection with manufacturing tolerances present in the various parts and the possibility that tolerance mis-matching can build up variations from the ideal geometric configuration. As a consequence of the possible mis-matching of tolerances in the parts making up a finished vehicle provision is made for mechanically adjusting wheel positions relative to the chassis or body of a vehicle. In some vehicles all adjustments are found in the steerable wheel assemblies, while in others the adjustments are provided in both the steerable and non-steerable wheel assemblies. Generally vehicles are permitted to have some deviations from the ideal conditions of wheel alignment and wheel to body alignment. As long as the deviations are not regarded as serious the vehicle is put into use.
In the vast majority of cases vehicles are sold with the alignment characteristics well within specification tolerance, and the alignment provides satisfactory results for the owners. Nevertheless, after a period of use the mechanism mounting the wheels on the chassis or body wear, and the alignment characteristics deviate from normal tolerance. Of course, under such conditions the provisions made for adjusting wheel alignment are used to restore the alignment to the desired conditions. A well treated vehicle can have its alignment diagnosed and adjusted to specifications, but one that has been mintreated or has been involved in a collision is much more difficult to diagnose or correct.
The many conditions attached to the original assembly of vehicles, and the equally many things that can occur to change wheel alignment on vehicles in use, make the design of apparatus to diagnose those conditions and changes very difficult. In the past apparatus has been provided that is capable of limited wheel alignment diagnosing ability. Some apparatus is simple to operate and some is very difficult and complicated, but in either type of apparatus there is little or no provision for obtaining a complete understanding of the interrelations of the steerable to non-steerable wheels, or of the relationship between wheels and vehicle body, or of the part that center point steering wheel position plays in relation to the other characteristics.
Examples of apparatus devised for examining vehicle wheel alignment characteristics include Carrigan U.S. Pat. No. 2,601,262, issued June 24, 1952, which is a light beam projection system incorporating passive light reflective mirrors on the steerable and non-steerable wheels and a chart spaced away from the vehicle to be in alignment with the light beam. Wheel alignment testing equipment involving placing it against machined surfaces on the wheels is shown in Holub U.S. Pat. No. 2,972,189 issued Feb. 21, 1961. This equipment is directed to the capability of locating and measuring misalignment in the frame and front and rear housings of a vehicle, as well as checking wheel tracking, all with line of sight telescopes and mechanical components. A somewhat similar vehicle wheel alignment device has been disclosed in MacMillan U.S. Pat. No. 3,091,862 issued June 4, 1963, but this is limited to use of sighting tubes and portable targets.
More sophisticated apparatus for indicating wheel alignment characteristics has been shown in Manlove U.S. Pat. Nos. 3,164,910 issued Jan. 12, 1965 and 3,181,248 issued May 4, 1965 respectively. In these arrangements mechanical pointers are caused to move over scales for checking alignment characteristics. The use of light beam projecting means for wheel alignment checking is disclosed by Holub U.S. Pat. No. 3,337,961 issued Aug. 27, 1967. However, the projectors illuminate scales at different times and so avoid simultaneous operation. Another system for using light beams in association with steerable wheels has been disclosed by Senften U.S. Pat. No. 3,782,831 issued Jan. 1, 1974 in apparatus for determining the angular position between a fixed and a movable body, such as the position of the steerable wheels relative to the axle. Electronic scanning devices butted against the vehicle steerable and non-steerable wheels has been disclosed by Hirmann U.S. Pat. No. 3,855,709 issued Dec. 24, 1974 wherein the scanning devices measure chassis geometry from the wheel position.
Alignment equipment limited to front wheel toe is shown in Butler U.S. Pat. No. 3,865,492, issued Feb. 11, 1975. This type of equipment has severe limitation as to what information can be obtained about the alignment characteristics of vehicle wheels and body. The prior art includes the electronic run-out compensation means of Senften U.S. Pat. No. 3,892,042 issued July 1, 1975 which is concerned with getting wheel alignment checking means properly oriented with the plane of wheel rotation. More recently, electronic alignment apparatus for indicating front wheel toe has been disclosed by Rishoud, et al U.S. Pat. No. 3,963,352 issued June 15, 1976, or by Florer, et al in U.S. Pat. No. 4,095,902 issued June 20, 1978, or by Lill in U.S. Pat. No. 4,097,157 issued June 27, 1978.